Monday, August 1, 2016

The US military attaché double transposition cipher

Apart from
the standard cryptosystems (Military Intelligence Code, War Department
Confidential Code. M-138-A strip cipher) US attaches also had an emergency
double transposition cipher. According to the instructions for this system,
found in the files of Pers Z (decryption department of the German Foreign
Ministry) (2):

Use of cipher.
To enable M/As to exchange safely secret or confidential messages with other attaches
or with assistants or agents acting under their direction, the double
transposition cipher is prescribed.

Keys.
The keys will be determined by the M/A. They will consist of short phrases
consisting of from five to twenty letters. They will be changed at frequent
intervals.

However the
same numerical sequence was used for both cages, which means that this system
would have been vulnerable to cryptanalysis.